Image Resolution for Digital Photography

Making Sharp Prints

At what image size must you record your photos in order to make sharp
prints? That depends on how large the image will be displayed and
from what distance it will be viewed. In order for an image to appear
sharp, individual pixels needs to be displayed small enough that they
appear to be points rather than small disks. For most humans
viewing an image from about 12" away, the maximum size of a "sharp
point" is about 250 microns (1/4mm or 1/100"). Doubling or halving
the viewing distance means doubling or halving the maximum pixel size
to retain sharpness. Small photos, like Christmas cards, are often
viewed from a much closer range, requiring a considerably smaller
point size (ie, higher
resolution).

For large prints hanging in the foyer which are rarely viewed from
closer than 12", this means that a resolution of 100 dpi (dots per
inch) is
probably good enough, and 150 dpi or 200 dpi is more than enough.
For a 16x24" print, the printer then requires an image size of 1600x2400
(3.8 MP) to maintain sharpness. Increasing the resolution to
150 dpi or the print size to 24x36" requires an image of 2400x3600
(8.6 MP). While 3.8 MP may seem very forgiving given today's sensor
sizes, remember that this allows no room for cropping the image, and
100 dpi provides barely acceptable sharpness when printed. If you
plan to crop your image, remember to take the new image dimensions
into account when calculating your maximum printable size.

Note that in order to make a noticeable improvement in resolution
(41%), you have to double the number of pixels on your camera's sensor.
If a 4 MP camera can produce a 100 dpi print, an 8 MP
camera can only produce a 140 dpi print of the same size, and a
16 MP camera is required to produce a 200 dpi print.

Here's another tip regarding the printing process. Most modern photo
printers operate at resolutions in the 1200-4800 dpi range. These
are far higher pixel dimensions than your camera can produce at
anything but a wallet size. In order to feed the printer the pixels
it's expecting, software somewhere along the way will fill in the
extra spaces between the pixels you provide with similar pixels so
that what hits the paper resembles what's on your screen. There are
different places where this replication could take place. You could do it
yourself in Photoshop before you print the image, or it could happen
in Photoshop's "print" back end, or in the printer drivers. The
latter stages are definitely preferred, as they will produce the
best results possible. Attempting to resize the image yourself
before printing by modifying the resolution can do no better than
letting the drivers handle it automatically, and if you don't know
what you're doing, you're more likely to make it look much worse.
Just let the printer drivers do their job and don't try to interfere
by forcing the resolution to something larger than what you've got.

If you do need to resize an image for whatever reason, you will
likely be given the choice of several different methods for filling
in the spaces between the original pixels, known as "interpolation
algorithms." "Pixel replication" or
"nearest neighbor interpolation"
simply duplicates the original pixels, and tends to result in blocky images.
"Linear" interpolation
gradually changes the RGB values between one original pixel and the
next. This is fast, and produces decent results.
"Bi-cubic" interpolation
does a better job with these gradual changes. It's the slowest method,
but produces the best looking results. Significantly enlarging an image
like this will still produce a blurry image, but it's the best you
can hope for, and will look fine from a distance.

Making Sharp Images

Keep in mind, too, that just because your camera has enough resolution
to produce a sharp print doesn't mean that the photos you take will
look sharp. Unless you employ good techniques to reduce blur, a photo
that looks fine on your camera's tiny LCD or even when scaled to fit
your computer monitor may still be quite blurry when viewed at 100%
magnification. That blur will be even more evident in a large print.
Fortunately, there are a number of ways to reduce image blur (or to
create it, if you're a "glass half empty" kind of person).

First, when recording JPEG images, always set your camera and software
to the highest quality settings possible. These may be labeled as
"super-fine" on your camera or "95-100%" in your computer software.
JPEG is a
lossy compression algorithm,
which means that every time you
edit and save an image, you loose detail. This typically shows
up as noise or blockiness. The lower the quality at which you encode
the JPEG, the more information you'll lose. Lower quality images require
less space on disk, which makes them ideal for sending through email or
uploading to a web page as long as you never expect to print them.
If you plan on making multiple edits to an image, it's best to save
interim copies of the image in a lossless file format such as
TIFF.
TIFF's "LZW" compression
algorithm can provide smaller file sizes that aren't much larger than
those of high-quality JPEGs, but without any loss in image quality like
you'd see in a JPEG. Incidentally, the old GIF file format also uses
this same LZW compression algorithm. The new PNG format which is gaining
popularity for web images is also lossless, but users a different algorithm.
You can edit an LZW-compressed
TIFF image as often as you like with no degradation in image quality.
TIFF is an old, well-established file format that any professional
printer and most consumer software will be able to handle just fine.
Canon's CR2 raw format is, in fact, just a TIFF file with a couple
of extensions.

Another way to improve image sharpness is to pay attention to your
shutter speed and make sure it's fast enough to eliminate any visible
shake from unsteady hands. For an average person using a camera with
no image stabilization, the slowest shutter speed at which you can
reliably produce sharp handheld images is 1 over your
focal length (1/50s for
a 50mm lens, 1/300s for a 300mm lens). Note that the focal length
mentioned is the effective 35mm focal length. Most low-end digital
SLR's (those using APS-C or "crop" sensors) must multiply the lens' true
focal length
by 1.5 or 1.6 to get the effective focal length. Point & shoot cameras
have considerably smaller sensors, and must thereby multiply the
printed focal length by 5 or 6 times to get the 35mm equivalent. Check
your owners manual for the actual number.
Image stabilization will
generally allow you to use a shutter speed that's 4-8 times slower
than the above number before you start getting blur from camera shake.

One final method for improving image sharpness is to shoot at a medium
aperture, typically 2-4
stops below "wide open" for a given lens. All lenses suffer from
distortion known as "spherical abberation" at wide open (numerically
small) apertures, while a different form of distortion known as "diffraction"
starts to take over at small (numerically large) apertures. Crop sensor
SLR's can get
diffraction-free shots down to f/16 before an 8x12" print starts to
get blurry. Point & shoot cameras can often only go to f/4 or f/5.6.
(This is why landscape photographers prefer cameras with the largest
sensors they can afford.)
Bob Atkins has written several
good articles with more information on this, like
this one.

Oh, and never use
digital zoom
if your camera has it, because it merely does a crop and resize,
but usually using a fast (poor quality) interpolation
algorithm. Just zoom as far as you can optically
and then crop if necessary on your computer.

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